The present invention is related to a method for cleaning electrical discharge ozone generators. More particularly, the present invention relates to circulating a warm fluid within the ozone generator to evaporate solid deposits of nitrogen oxides, including dinitrogen pentoxide, from the tubes and dielectrics of the generator.
Ozone (O3) is a strong oxidizing agent (2.07V) that is used as a disinfectant in various applications, such as wastewater treatment, cooling towers, air treatment, swimming pool cleaning, food processing, hydroponics, and meat processing. Ozone is particularly effective in aqueous environments. Ozone is, however, very reactive and cannot be stored for any significant period of time. As a result, ozone must be generated at the site where it is to be used. Two common means by which ozone is generated are by subjecting oxygen gas (O2) to ultraviolet radiation or to an electrical discharge.
One type of electrical discharge generator is an electrical barrier discharge ozone generator, commonly known as a silent discharge generator. One such generator is a corona discharge generator. Corona discharge generators are commonly used to generate ozone on a large scale. The basic principle of electrical discharge ozone generators is that a feed gas is fed through a high voltage electrical discharge field between two electrodes. The oxygen is then ionized as it passes through the electrical field which will cause at least some oxygen to be converted to ozone. For corona discharge generators, the feed gas, usually dry air or oxygen, is subjected to coronal discharges created by high voltages between two electrodes, one of which is contained within a dielectric material. In a tubular ozone generator, a dielectric is supported within a tube and a central cathode within the dielectric is subjected to a high voltage relative to an outer anode. The anode is often grounded. High voltage phenomena occur inside the dielectric envelope and induce a corona discharge field between the outside of the dielectric envelope (hereinafter referred to as xe2x80x9cdielectricxe2x80x9d) and the outer anode material. An oxygen-containing feed gas is fed into this space and through this field, and the oxygen (O2) molecules are split to form atomic oxygen, which then reacts to form ozone.
3O2+Energyxe2x86x922O3xe2x80x83xe2x80x83(Eq. 1)
The quantity of ozone generated depends on several factors, such as for example the voltage, the frequency of AC current, the gap between the dielectric and the cathode and the concentration of O2 and other gases in the feed gas. The feed gas may be dry, clean air; dry, clean oxygen; or dry, clean oxygen containing small amounts of other relatively inert gases such as nitrogen (N2) or argon (Ar). It is important that dry feed gas be used, as water interferes with the reaction and also reacts with gases in the ozone gas to create contaminants, most notably nitric acid (HNO3).
Much of the energy required for the reaction is lost as heat; therefore ozone generators should be cooled to operate more efficiently. One of the ways that cooling the generator increases the efficiency of the generator is by causing fewer O3 molecules to be lost due to decomposition or collision. A good description of ozone generating equipment can be found in U.S. Pat. No. 4,954,321 by Jensen issued Sep. 4, 1990, and also in xe2x80x9cOzone Technology and Equipment Designxe2x80x9d, Ozonia North America, USA 1996, the information in, and contents of, both documents hereby being incorporated herein by reference.
Large amounts of ozone are not easily generated. For example, an ozone generation system employing an electrical discharge and which uses liquid oxygen at xe2x89xa799.5% purity, that has been vaporized and has had between 2 and 3% N2 by weight and a certain amount of argon added, will typically produce 10-13% ozone by weight. Because of the relatively low rate of ozone generation, large plants may require several ozone generators to meet the demand for ozone. In turn, each generator may contain many anode/cathode/dielectric units.
As noted above, the amount of ozone generated depends on several factors, one of which is the amount of N2 in the feed gas. When oxygen separated from air is used as a feed gas, nitrogen may be present. This is because of the method used to separate the oxygen from the air, e.g. vacuum or pressure swing adsorption or cryogenic separation. Nitrogen may also be present in the feed gas because it has been introduced to act as a catalyst. Nitrogen allows production of a higher ozone concentration or the reduction of the power consumed in generating the ozone. For example, large commercial ozone generators using pure oxygen generally create between 6-10% ozone by weight, instead of the 10-13% available when a small amount of N2 is added. It is therefore not desirable to remove all of the N2.
Unfortunately, it has been discovered that the presence of nitrogen in the feed gas results in a solid residue, mainly composed of dinitrogen pentoxide (N2O5), with some of it being deposited within the generator system, including on the tubes and the dielectrics. The residue may also contain other solid oxides of nitrogen (NOx). The oxide deposits on the support tubes and dielectrics and may eventually clog the passageway between the dielectric and the support tube.
Regular maintenance of ozone generators typically involves an inspection and repair of the electrical connections. However, because of the problems inherent in cleaning the generators described in greater detail below, opening the ozone generator to the atmosphere is avoided whenever possible. From time to time, however, ozone generators may require special or preventative maintenance. Such maintenance may be occasioned by failure of more than approximately 10% of the dielectrics or by deposits that clog the passages between the dielectrics and their support tubes in some systems.
Current methods of cleaning ozone generators consist of turning off the power supply and cooling water and purging the generator by circulating dry oxygen gas at room temperature through the system. The purging continues until the residual ozone has been removed from the inside of the generator for the safety of the workers. Thereafter the system is opened up to the atmosphere.
When the ozone generator is opened for regular maintenance, if it is opened for long enough, the water in the ambient air reacts with any residual solid nitrogen oxides to form nitric acid (HNO3). The reaction with N2O5 for example, proceeds as follows:
H2O+N2O5xe2x86x922HNO3xe2x80x83xe2x80x83(Eq. 2)
Nitric acid is an oily, yellow residue, and any nitric acid in the generator needs to be removed.
The cleaning typically requires that all dielectrics and the tubes holding them be cleaned with a proper solvent. Generally, the dielectrics and tubes are removed from the system, cleaned with water and then with an industrial organic solvent such as acetone or a chlorinated organic solvent such as perchloroethylene, or methanol. This cleaning work is time consuming, and may require more than 14 days for an industrial scale ozone generator. In addition, removal and cleaning of the dielectrics will result in some breakage (perhaps 10%), thereby requiring their replacement. Finally, the chlorine containing solvents and the disposal of the contaminated cleaning solvents represent additional cost and safety issues that must be considered.
It is therefore desirable to have a less onerous cleaning method that would decrease the time and expense required for special maintenance of electrical discharge ozone generators, particularly large-scale corona discharge ozone generators.
The current invention relates to a method of removing solid deposits of the oxides of nitrogen, including in particular dinitrogen pentoxide, in an ozone generator thereby avoiding the need to open the generator to atmosphere. If it is necessary to open the generator, to replace dielectrics for example, the inventive method will significantly reduce or eliminate the creation of nitric acid residue within the system. The inventive method can significantly reduce the maintenance time required from perhaps two to three weeks for each generator to perhaps as little as three days. In addition, damage to the dielectrics is minimized or eliminated, as is the need for solvents to remove the nitric acid. The method can provide significant costs savings in personnel time and materials.
A preferred embodiment of the inventive method uses warm gas circulation, preferably at 47-65xc2x0 C., within the generator dielectric support tubes and warm water circulation in the shell section of the generator, preferably at 47-65xc2x0 C. If the physical components of the generator can withstand temperatures above 65xc2x0 C. then the temperature of the gas can be increased well above 65xc2x0 C., although this is not necessary to remove dinitrogen pentoxide, and heating the gas to a higher temperature may make the cleaning process more expensive. The fluid circulation within the system raises the temperature within the generator, and various solid oxides of nitrogen, which have boiling points less than the temperature of the circulated gas, including dinitrogen pentoxide which has a boiling point of about 47xc2x0 C., are evaporated and thereafter evacuated from the system by the gas stream. The temperature within the generator is sufficient to ensure that the deposits do not re-form within the ozone generator.
The progress of the cleaning can be monitored by bubbling a portion of the evacuated gas through a water trap and measuring the change in pH caused by the HNO3 formed by interaction of the NOx and the water. Fluids are circulated within the generator until the pH of the water used as a reference is not appreciably lowered by the gas exiting the tubes of the generator.
If the need for maintenance was caused only by a build-up of NOx, including in particular N2O5, the system is ready to return to production without requiring the generator to be opened to the atmosphere. Cleaning time and potential contamination are reduced. If the maintenance was required because of damaged dielectrics, when the system is opened to ambient air after being sufficiently cleaned, no nitric acid is formed. Only those dielectrics requiring replacement need be removed and replaced. Again, there are significant benefits in terms of both time and material savings.
In one aspect of the invention there is provided a method of cleaning an electrical discharge ozone generator comprising passing a warm cleaning gas between an inlet of the generator and an outlet of the generator to evaporate at least some of the NOx deposited in the ozone generator.
In another aspect of the invention there is provided a method for removing solid deposits of NOx from an ozone generator comprising a first and second electrode, the electrodes being spaced from each other and having a passageway therebetween. The solid deposits of NOx are located within the passageway. The method comprises the step (i) of passing a warm cleaning gas through the passageway to evaporate the solid deposits of NOx with boiling points equal to or less than 65xc2x0 C. which are deposited therein. The warm cleaning gas exiting the ozone generator is at a temperature sufficient to maintain the NOx in a gaseous state until the NOx exits the ozone generator.
In another aspect of the invention there is provided a method for removing solid deposits of NOx from an ozone generator comprising a housing enclosing an interior having an inlet and an outlet and a pair of spaced electrodes mounted within the interior. The electrodes are spaced apart from each other. The solid deposits of NOx are located within the interior. The method comprises the step of passing a warm cleaning gas through the interior from the inlet to the outlet to evaporate at least some of the NOx deposited therein. The warm cleaning gas exits the ozone generator at a temperature sufficient to maintain the NOx in a gaseous state until the NOx exits the ozone generator.
In a further aspect of the invention there is provided a method for removing solid deposits of NOx from an ozone generator comprising a housing and a plurality of support tubes mounted within the housing. The support tubes each support one or more dielectrics and each of the support tubes has an inner wall. A passageway is formed between the inner wall of the support tubes and the dielectrics. The passageway has solid deposits of NOx therein. A support tube inlet is in flow communication with a support tube outlet through the passageway. The method comprises the step (i) of passing a warm cleaning gas through the passageway to evaporate at least some of the solid deposits of NOx which are deposited therein and carry at least some of the evaporated NOx from the ozone generator.
In another aspect of the invention there is provided a method for removing solid deposits of NOx from an ozone generator comprising an outer housing and a plurality of support tubes mounted within the housing. The support tubes each support one or more dielectrics and each of the support tubes has an inner wall and a passageway between the inner wall and the dielectrics. The passageway communicates between a support tube inlet and a support tube outlet. The housing has a shell that defines an interior surrounding the support tubes, the interior communicates between a shell inlet and a shell outlet. The method comprises step (i) of circulating a warm fluid within the shell and the concurrent step (ii) of evacuating the support tubes to remove the evaporated NOx with boiling points less than 65xc2x0 C. that had been deposited therein.
In a further aspect of the invention there is provided a method for removing solid deposits of NOx from an ozone generator comprising an outer housing and a plurality of support tubes mounted within the housing. The support tubes each support one or more dielectrics and each of the support tubes has an inner wall and a passageway between the inner wall and the one or more dielectrics. The passageway communicates between a support tube inlet and a support tube outlet. The housing has a shell that defines an interior surrounding the support tubes. The interior communicates between a shell inlet and a shell outlet. The method comprises step (i) of circulating a cleaning gas within the support tubes and concurrent step (ii) of circulating a warm fluid within the shell to heat the cleaning gas, thereby removing the NOx with boiling points less than 65xc2x0 C. deposited therein. The temperature of the warm fluid is sufficient to ensure that the temperature of the cleaning gas exiting the ozone generator is sufficient to maintain the NOx in a gaseous state until the NOx exits said ozone generator.
In yet another aspect of the invention there is provided a method for removing dinitrogen pentoxide deposits from an ozone generator comprising an outer housing and a plurality of support tubes mounted within the housing. The support tubes each support one or more dielectrics and each support tube has an inner wall and a passageway between the inner wall and the dielectrics. The passageway communicates between a support tube inlet and a support tube outlet. A shell surrounds the support tubes, the shell defining an interior surrounding the support tubes. The interior communicates between a shell inlet and a shell outlet. The method comprises circulating a clean, dry mixture of oxygen, nitrogen and argon at 55xc2x0 C.-60xc2x0 C. between the shell inlet and shell outlet; supplying the shell with warm water at 55xc2x0 C.-60xc2x0 C.; diverting a portion of the gas exiting the support tubes to a liquid ring compressor; adding a neutralizing agent to the water in the compressor to maintain the pH in the liquid ring compressor at an approximately constant pH using an in-line process pH controller; and continuing the cleaning until the addition of neutralizing agent terminates as it is no longer required to maintain the constant pH.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific, preferred embodiments of the invention in conjunction with the accompanying figures.